In such high-tension electric installations, a high-intensity fault arc, e.g. of several tens of thousands of amps lasting a few tenths of a second, develops a high pressure due to heating inside the casing. The destructive effects of this pressure can be avoided by safety valves or diaphragms. Under the effect of the rise in the gas pressure due to this heating the arc tends to become stabilized at some points, and in particular near insulators, once the arc reaches them. The heat at the base of the arc then locally weakens the casing by melting. This phenomenon combined with the pressure may lead to perforation of the casing. To avoid this, it is known, e.g. through published French Pat. No. 2,217,839 and corresponding U.S. Pat. No. 3,931,451, to provide electrodes made of arc-resistant material at the points where the arc fixes itself. Said materials are not subjected to the pressure and therefore protect the casing.
Another means consists in making the arc rotate rapidly so as to reduce local heating. On this subject, reference should be made to published French patent No. 2,403,669 and corresponding U.S. Pat. No. 4,090,028 which uses the magnetic field set up by one or several turns which are electrically insulated except at one end which is connected to the casing in a zone where the arc is prevented from moving.
However, this kind of protection requires bulky devices at the cost of smaller insulator diameter or increased insulator dielectric stress.
Preferred embodiments of the present invention provide a compact device which is simple to manufacture and sets up an impenetrable barrier against arcs, making the arcs unstable and thereby preventing holes from being formed in a casing.